Loop certification and measurement for ADSL

ABSTRACT

The present invention involves a transmission of a known signal over a line intended for a digital subscriber line (xDSL) service, measurement of a resulting waveform received through the line and processing of the received signal to derive a set of eigenvalues characterizing the transfer function of the line. In the preferred embodiment, a transmitter applies a known test signal waveform to the transmission line. The test signal corresponds to a desired one of several available xDSL services. At the receiving end of the line, the output signal from the line is sampled, to capture a digitized sample waveform for the received signal. A computer processes the captured waveform, to determine eigenvalues characterizing the transfer function of the line with respect to the one service. If the eignevalues for the line bear predetermined relationships to threshold values, indicating acceptable line transfer function for the desired xDSL service, then the invention certifies the tested line for use on that service.

TECHNICAL FIELD

The present invention relates to a technique for measuring theperformance of wire pairs used for high speed digital subscriber line(xDSL) services, for example for Asymmetrical Digital Subscriber Line(ADSL) services and the like. The measurement technique enables anetwork carrier to test a line, typically a twisted-wire copper pair, todetermine if the line characteristics will permit a particular level ofdigital service. The carrier can then certify the line for that service.

ACRONYMS

The written description uses a large number of acronyms to refer tovarious services and system components. Although generally known, use ofseveral of these acronyms is not strictly standardized in the art Forpurposes of this discussion, acronyms therefore will be defined asfollows:

ADSL—Asymmetrical Digital Subscriber Line

ATM—Asynchronous Transfer Mode

ATU-C—ADSL Terminal Unit-Central Office

ATU-R—ADSL Terminal Unit-Remote

CO—Central Office

CP—Customer Premises

CPU—Central Processing Unit

DSL—Digital Subscriber Line

DSLAM—Digital Subscriber Lim Access Multiplexer

EEPROM—Electronically Erasable/Programmable Read Only Memory

EOC—Embedded Operations Channel

HDSL—High data rate Digital Subscriber Line

I/O—Input/Output

IP—Internet Protocol

ISDN—Integrated Services Digital Network

LAN—Local Area Network

LCD—Liquid Crystal Display

MDF—Main Distribution Frame

MLT—Mechanized Loop Test

MPEG—Moving Pictures Experts Group digital encoding

NID—Network Interface Device

PC—Personal Computer

POTS—Plain Old Telephone Service

QoS—Quality of Service

R—Ring

RAM—Random Access Memory

ROM—Read Only Memory

SMDS—Switched Multi-Megabit Data Service

T—Tip

TCP—Transmission Control Protocol

TLS—Total Least Squares

VDSL—Very high data rate Digital Subscriber Line

xDSL—Generic class of Digital Subscriber Line Services

BACKGROUND

Modem society continues to create exponentially increasing demands fordigital information, and the communication of such information createsincreasing needs for ever-faster data communication speeds. The mostcommon form of computer-to-computer communication in use today relies onmodems and analog telephone network connections. The telephone network,however, was designed to provide approximately 3.3 kHz of analog voicebandwidth. Such a bandwidth provides adequate voice communication, atlow cost, but does not support high-speed data communications.Integrated Services Digital Network (ISDN) offers somewhat faster datacommunications and the capacity for concurrent data and voice telephoneservices. However, this technology has some drawbacks, and data ratesoffered by ISDN already may be too slow. The high-speed and wideavailability of modern personal computers (PCs) continually gives riseto ever more sophisticated multimedia applications. Communications forsuch applications, typically between the PC and the Internet, aredriving the need for speed to rates far above those available on analogtelephone lines and on normal ISDN lines.

A number of technologies are being developed and are in early stages ofdeployment, for providing substantially higher rates of datacommunication, for example ranging form 640 kb/s to 9 Mb/s. Ofparticular note, after considering several other options, a number ofthe local telephone carriers are working on enhancements to theirexisting copper-wire loop networks, based on various xDSL technologies.xDSL here is used as a generic term for a group of higher-rate digitalsubscriber line communication schemes capable of utilizing twisted pairwiring from an office or other terminal node of a telephone network tothe subscriber premises. Examples under various stages of developmentinclude ADSL (Asymmetrical Digital Subscriber Line), HDSL (High datarate Digital Subscriber Line) and VDSL (Very high data rate DigitalSubscriber Line).

Consider ADSL as a representative example. For an ADSL related service,the user's telephone network carrier installs one ADSL modem unit at thenetwork end of the user's existing twisted-pair copper telephone wiring.Typically, this modem is installed in the serving central office or inthe remote terminal of a digital loop carrier system. The user obtains acompatible ADSL modem and connects that modem to the customer premisesend of the telephone wiring. The user's computer connects to the modem.The central office modem is sometimes referred to as an ADSL TerminalUnit—Central Office or ‘ATU-C’. The customer premises modem is sometimesreferred to as an ADSL Terminal Unit—Remote or ‘ATU-R’.

For digital data communication purposes, the ATU-C and ATU-R modem unitscreate at least two logical channels in the frequency spectrum abovethat used for the normal telephone traffic. One of these channels is alow speed upstream only channel; the other is a high-speed downstreamonly channel. Two techniques are under development for dividing theusable bandwidth of the telephone line to provide bidirectionaltransmission. Currently, the most common approach is to divide theusable bandwidth of a twisted wire pair telephone line by frequency,that is to say by frequency division duplexing. The frequency divisionapproach uses one frequency band for upstream data and another frequencyband for downstream data The downstream path is then divided by timedivision multiplexing signals into one or more high-speed channels andone or more low speed channels. The upstream path also may betime-division multiplexed into corresponding low speed channels. Theother approach uses Echo Cancellation. With Echo Cancellation, theupstream band and the downstream band substantially over-lap. The modemsseparate the upstream and downstream signals by means of local echocancellors, in a manner similar to that used in V.32 and V.34 modems.

The telephone carriers originally proposed use of ADSL and similarhigh-speed technologies to implement digital video services, for examplein networks sometimes referred to as video ‘dialtone’ networks. The ADSLline technology provided a mechanism for high-speed transport of MPEGencoded video information to video terminal devices in the customers'homes. Examples of such ADSL-based video dialtone networks are disclosedin U.S. Pat. Nos. 5,247,347, 5,410,343 and 5,621,728. Interest in suchvideo services has waned, but the recent explosion in Internet and otherPC-based services has sharply rekindled the carriers' interest in xDSLtechnologies. The carriers are now proposing a range of xDSL dataservices targeted at high-speed Internet access and high-speed access toprivate data networks. U.S. Pat. No. 5,790,548 to Sistanizadeh et al.discloses an example of an ADSL based data network, e.g. for high-speedaccess to the Internet and to corporate LANs.

In the last year or so, considerable attention has focused on oneversion of ADSL with somewhat reduced capabilities but which does notrequire a separate splitter/combiner at the customer premises tosegregate the telephone traffic from the data traffic. The ADSL ‘Lite’modem can plug directly into the customer's telephone wiring, without aspecial installation by a telephone company technician. The customer's‘Lite’ modem does not need or include a frequency splitter/combiner tosegregate the voice and data traffic. The ‘Lite’ modem uses a morerestricted frequency band, in order to reduce interference withtelephone service. Although this reduces the downstream data ratesomewhat, particularly for longer lines, the ‘Lite’ implementation stillprovides downstream speeds ranging from 640 b/s to 1.5 Mb/s, which aresubstantially higher than provided by analog modems or ISDN.

Thus, ADSL modems today are providing downstream data rates in rangesfrom 640 kb/s to as high as 9.1 Mb/s. The precise data rate depends onmany factors, such as line length, copper wire gauge. cross-coupledinterference, and the like. As a general rule, the shorter the distance,and/or the smaller the wire gauge, the higher the rate can be on theparticular telephone line. These rates provide an order of magnitudeimprovement over telephone line modems and ISDN equipment currently usedfor Internet or other data network access services.

Installation, operation and maintenance of ADSL-based data services,however, pose a number of problems. These problems may be particularlyacute where a carrier is considering upgrading service to ADSL on anexisting subscriber's line circuit. As noted, the length and gauge ofthe wiring can effect performance. If the wiring has been in place andused for telephone service, there may be load coils on the line, whichdisrupt xDSL services. Bridged-taps, which are common in telephone loopplant, also cause performance problems.

In the telephone industry, twisted wire pair circuits from a centraloffice or a subscriber line carrier unit generally are bridged-tappedalong their length, to provide a line appearance in a number ofdifferent terminals located at different points along the multi-pairfeeder cable. An installer can connect a subscriber's drop line tobinding posts in the closest terminal, but the line appearance remainsin other terminals connected to the multi-pair cable. At a later date,an installer can disconnect the first subscriber drop line from the oneterminal, and connect a new subscriber's drop line from anotherterminal, in order to reuse the twisted wire pair connection through thefeeder cable back to the central office for another subscriber. Thepresence of bridged-tapping, particularly extended wiring downstream ofa particular subscriber's connection to twisted wire pair in a terminal,may cause considerable disruptive interference effects. For example, theextending wiring adds capacitance and resistance. The extended wiringpicks up considerable electromagnetic interference from external sourcesand may pick up cross talk from adjacent active pairs. All of theseeffects disrupt xDSL broadband digital service on the twisted wire pair.

To install and operate an xDSL modem on a line, the telephone carrierneeds to know if the line is in such a state as to enable high-speeddownstream transmission at the full rates desired for the particularxDSL service. If not capable at the highest rate, it is usefull todetermine what lower rate the line may support. The carrier company cantake a number of different steps, if it knows the line capabilities. Forexample, the carrier may rearrange its line connections to provide asubscriber desiring a particular xDSL service with an adequate linepair.

At present, however, there is no adequate technique for testing loopplant wiring, particularly existing telephone wiring, for itscompatibility with the high-speed data services. Often, the carrier mustinstall the modems for the desired service, operate the modems and praythey work. If there are problems, there is no easy way to measure ordiagnose specific problems. The carrier's technicians can try a numberof different fixes, on a hit-or-miss basis, and retest the modemoperations. Such approaches to testing are time consuming and oftenineffective.

Many carriers today utilize a mechanized loop test (MLT) system, foranalyzing reported troubles on subscribers' telephone lines. An MLTsystem selectively connects to the central office terminals of twistedpair telephone wiring and conducts electrical tests on metalliccircuits. Such a system can apply an AC voltage across a wire pair,between the Tip (T) wire and ground, and between the Ring (R) wire andground and take appropriate measurements to determine characteristicimpedances. The MLT system can also measure the DC resistance betweenthe wires and between each wire and ground. The MLT system stores a listof DC and AC resistance/impedance values that correspond to certain lineconditions, e.g. shorts, opens, normal telephone set connections, etc.The MLT system makes decisions as to presence or absence of differenttypes of faults by comparing the test result values to its stored listof values. These MLT tests provide limited information regarding thetransfer characteristic of the loop, particularly with respect to thefrequency ranges effecting xDSL services.

A need therefore exists for an efficient technique for testing a line tocertify the line for a particular high-speed data service, like ADSL,before putting the line into service. A need also exists for a techniqueto enable testing and maintenance of in-service lines, to enable thecarrier to respond to troubles and outages reported by subscribers.

SUMMARY OF INVENTION

The present invention addresses the above stated needs by providingtransmission line measurements and processing of measurement results, todetermine at least one eigenvalue and preferably a plurality ofeigenvalues, for a line intended for a digital service. The eigenvaluescharacterize the transfer function of the line as a data communicationmedium. Comparison of the one or more values to one or more standardthreshold values can be used to determine operability for a particulargrade of digital service.

Thus, a first aspect of the invention relates to a method of testing atransmission line for a selected one of several digital subscriber lineservices. The method includes selecting one waveform, corresponding tothe particular digital subscriber line service, from waveformscorresponding to a number of such services. The selected waveform istransmitted through the line to a receiving end of the line. The testinvolves sampling and capturing a waveform from the receiving end of thetransmission line and processing samples of the captured waveform toproduce a set of eigenvalues characterizing a transfer function of thetransmission line.

The preferred embodiment compares each of the eigenvalues (possiblecomplex) to a predetermined (possibly complex) threshold value. Thethresholds represent an acceptable transfer function of a transmissionline with respect to the selected digital subscriber line service. Ifeach of the eigenvalues bears a predetermined relationship to therespective threshold value, for example, if the eigenvalue exceeds thethreshold, then it is possible to certify the transmission line forcarrying the selected digital subscriber line service.

In a typical example, a customer would ask an exchange carrier for aparticular grade of digital service. The carrier's technician would testthe line, as outlined above. The testing produces a number ofeigenvalues. If the eigenvalues from the test bear the appropriaterelationship to corresponding thresholds associated with the particulargrade of digital service, the test indicates an acceptable condition ofthe line for that service. The carrier's technician can certify theline, and the carrier can provide the customer the digital service overthe particular line.

The inventive method may be applied to lines of a variety oftelecommunications networks that carry digital data services. Thepreferred embodiments utilize the inventive testing and certificationtechnique to certify telephone lines, typically twisted pair typesubscriber lines, for xDSL services. Examples of the xDSL servicesdiscussed in detail include asynchronous digital subscriber line (ADSL)services, such as the new splitterless ADSL service.

Another aspect of the invention involves a system for testing a line ofa communication network for a digital subscriber line service. Thesystem includes a test waveform generator, for connection to atransmit-side of the line. A waveform sampler, for connection to areceive-side of the line, samples a test waveform received through theline from the generator. A processor, coupled to the waveform samplerand responsive to the sample of the test waveform, determines a set ofeigenvalues. These values are representative of the transfer function ofthe transmission line. The processor compares the eigenvalues tothreshold values, which correspond to an acceptable transfer function ofthe line.

In the preferred embodiment, the waveform sampler and the processor areelements of a test unit coupled to the receiving end of the transmissionline, preferably on the network end of the line. A storage device storessets of threshold values for a number of services. The processorprocesses a set of samples for a test waveform corresponding to aselected one of the digital subscriber line services and processes thecaptured digital samples. From this processing, the processor typicallydetermines two or more eigenvalues representing the transfercharacteristic of the line with respect to the selected digitalsubscriber line service. The processor compares the eigenvalues to aselected set of threshold values. The selected set of threshold valuescorresponds to an acceptable transfer function for the selected digitalsubscriber line service.

The preferred embodiment of the test unit is built around a computersystem. The system includes a keyboard or the like for input as well asa display. Under software control from the processor, the keyboard anddisplay provide a user interface for a technician to control the testoperations. For example, the processor drives the display to showresults of the comparison of the eigenvalues to the selected set ofthreshold values. The displayed results show whether or not a line cansupport a desired digital subscriber line service. If so, then thetechnician can certify the line for the customer's use and subscriptionto the particular digital service. If not acceptable, the relationshipof the eigenvalues to the threshold values may provide informationuseful to the network operator, for example to facilitate repairs.

The testing of the invention may apply to transmissions in eitherdirection on a subscriber line. For an ADSL service, for example, thetechnician would at least test for the broadband communications, thedownstream transmission toward the customer premises. The transferfunction of twisted wire pair is the same in each direction, so thebroadband test actually may use a test signal transmission from thecustomer premises to a test unit on the central office or network end ofthe line. A disclosed implementation also tests the transfercharacteristic of the line with respect to the upstream transmission forthe ADSL service. Alternatively, for prescribed services. upstreameigenvalues might be inferred for a downstream eigenvalue.

The present invention thus provides an efficient, effective techniquefor testing a line of a communication network. The test providesinformation that specifically relates to the ability of the line totransport the signals for a selected digital subscriber line service.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF DRAWINGS

The drawing figures depict the present invention by way of example, notby way of limitations. In the figures, like reference numerals refer tothe same or similar elements.

FIG. 1 is a simplified functional block diagram of a system forcertifying a transmission line for an xDSL service by measuringeignevalues for the line, in accord with the invention.

FIGS. 2 and 2A to 2F are a series of graphs helpful in understanding acalculation of eigenvalues for loop testing.

FIG. 3 is a functional block diagram, depicting an example of a localcarrier's telephone and data network incorporating the line testing andcertification in accord with the present invention.

FIG. 4 is a functional block diagram, which illustrates the componentsof exemplary test units used to implement the invention, for example inthe network of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention involves a transmission of a prescribed signalover a line, measurement of a resulting waveform received through theline and processing of the transmitted and received signals to deriveeigenvalues representation of the transfer characteristics of the line.

At the transmit-end of the line, a known test signal waveform is appliedto the transmission line. The test signal corresponds to a desired xDSLservice. At the receive-end of the line, the output signal from the lineis sampled, to capture a digitized sample waveform for the receivedsignal. A computer processes the captured waveform, to determine thetransfer function of the transmission line. The waveform of the testsignal represents known parameters, and the relationship of the receivedsignal to the known parameters represents a measure of the impact of theline transmission characteristics on the known signal.

In mathematical terms, an operator “operates” on a function and producesanother function. For every operator, there is a collection of functionswhich, when operated on by the operator produces the same function,modified only by a constant factor. Such a function is called aneigenfunction of that operator. The constant factor is the eigenvalue ofthat eigenfunction.

In accord with the invention, the computer processes the capturedwaveform to determine at least one and typically a plurality ofeigenvalues, which are representative of the measured transfer functionof the tested line. If the eigenvalues for the line bear predeterminedrelationships to threshold values, indicating acceptable linecharacteristics for the desired xDSL service, then the inventioncertifies the tested line for use on that service. If the eigenvalues donot relate to the thresholds in the desired manner, the precisedivergence from the thresholds may provide information that is useful indiagnosing the problem(s).

FIG. 1 depicts a system for testing and certifying a line for xDSLservice, in accord with the invention. The simplified embodimentillustrated performs tests of the transmission characteristics in onedirection over the line, for example, for the broadband transmissionfrom the network side downstream to the customer premises, for an ADSLservice or the like.

The test system shown includes a first unit 10 at the transmit-end ofthe line 20 and a second unit 30 at the receive-end of the line 20. Inthis simple embodiment, the unit 10 is a transmitter, and the unit 30 isa receiver. For xDSL type testing operations, the transmission linetypically will be a twisted wire pair, formed of two insulated copperconductors. Persons skilled in the art, however, will recognize that theinventive testing may be applied to other lines or media that will carrydigital communications, such as coaxial cable links.

The transmitter unit 10 includes a signal generator 11. A memory 13stores waveform parameters for a number of available test signals. Eachavailable test signal corresponds to one of the xDSL services for whichthe system may perform line testing and certification. The transmitterunit 10 also includes a controller 15. The controller 15 controls theparameters supplied to the signal generator 11 and controls thegenerator, to enable selection of the actual waveform generated tocorrespond to a particular desired service.

For example, a technician might instruct the controller 15 to generate atest waveform corresponding to an ADSL service. The controller 15 causesthe memory 13 to supply the appropriate waveform parameters, and inresponse, the signal generator 11 outputs a precise test waveform. Thegenerator 11 is coupled to the transmission line 20, and the linecarries the test waveform to the receiver unit 30.

The receiver unit 30 includes a coupler 31. The coupler 31 providesphysical and electrical interconnection to the transmission line 20, toenable reception of the waveform signal from the particular medium ofthe line, for example, from a twisted wire pair. A digitizer 33 samplesthe waveform on the line, and waveform sampler 35 captures the digitizedsignals for a complete waveform representative of the signal receivedfrom the transmission line 20. The sampler 35 supplies the digitalwaveform signal to a computer 37, for further processing.

As discussed in more detail below, the computer 37 executes a procedureto calculate eigenvalues for the line transfer function from thereceived waveform. A preferred procedure is summarized below. Thechannelization theory and formulas involved in eigenvalue calculationsfor channel transfer functions are described in detail in Proakis,Digital Communications, 2^(nd) Edition, © 1988, McGraw-Hill, pp.551-578. An alternative procedure using the received waveform and asample of the test waveform is a total least squares (TLS) proceduresimilar to that disclosed in Rahman & Sakar, “Deconvolution and TotalLeast Squares in Finding Impulse Response of an Electromagnetic Systemfrom Measured Data,” IEEE Transactions on Antennas and Propagation, vol.43, no. 4, April 1995, pp. 416-421. The disclosures of the Proakis textand the Rahman and Sakar article are incorporated entirely herein byreference. Through either procedure, the computer 37 determines theeignevalues for the response characteristic of the line 20 that iscurrently under test.

If desired to facilitate TLS processing computer 37 may also receive asample of the original input waveform. This could be a measured sample,but in the illustrated embodiment, a memory 39 stores samples of theinput waveforms for the various xDSL services. During systemcalibration, the generator 11 generates the test waveform signals forall of the xDSL services. The generated test signals are digitized,sampled and stored in memory 39. Other procedures may be used to storeadequately accurate test waveform samples in the memory 38. During testprocessing, the computer 37 obtains the appropriate waveform sample fromthe memory 39.

The computer 37 also connects to a memory 41. The memory 41 storesthreshold eigenvalues representing of acceptable transfercharacteristics of transmission lines, for the different xDSL services.The thresholds are set based on laboratory testing using the associatedxDSL modems. To certify a line as acceptable for the desired xDSLservice, the computer 37 retrieves the threshold values for theappropriate service from the memory 41 and compares those values to theeigenvalues derived from the testing operation.

If the eigenvalues bear a desired relationship to the thresholds, forexample, exceed all of the thresholds, the computer provides anindication of acceptability (possibly for a presented Quality of Serviceor QOS) via a user interface 43. In a typical computer implementation,the user interface comprises a keyboard, a mouse or the like and adisplay device such as a CRT monitor. In such an implementation, thecomputer displays the certification results on the monitor.

One methodology for calculating the channel eigenvalues can beillustrated by considering the following much simplified example. Thisexample, conjured more the tractability than for realism, presupposesthat the unknown transmission line (loop) transfer function H(f) issinusoidal within the frequency band 0-100 KHz, as shown in FIG. 2A.Assume a need to test a narrow band of 40-50 KHz, for purposes of thissimplified example. The xDSL signal, and therefore so too the testwaveform, are presupposed (for this example) to have a raised-cosinepower spectral density strictly bandlimited between 40 and 50 KHz, asshown in FIG. 2B. Those skilled in the art will recognize that a broaderband test signal would be used to test a wider portion of the spectrum,e.g. to match one or more of the ADSL channels.

Calculation of the channel eigenvalues begins with the calculation ofthe autocorrelation function of the received signal at the output of thetransmission line. [Proakis, Equation (6.3.8)]. In the preferredembodiment, this function is calculated by summing appropriately laggedproducts at the output of a Sample. In this illustrative example, thecalculation is expedited by taking the (fast) Fourier transform of the(sample, low-pass equivalent, normalized) power spectral density at theoutput of the transmission line [Proakis, Equation (1.2.28)]. If thecalculations are further expedited by using only sixteen (2⁴) frequencysamples across the 10 KHz band, then the digitized output power spectraldensity essentially appears as shown in FIG. 2C and the autocorrelationappears as an FIG. 2D.

The autocorrelation function is assumed to be zero for |i−7|>L, where Lis some arbitrary positive integer. For this illustrative example, L hasbeen determined by setting the autocorrelation function to zero for allindices beyond the neighborhood of its maximum where the correlationfalls more than 10 decibels below its maximum for the first time.Consequently, L=3 the tap-gain coefficients of the equivalentdiscrete-time mode are shown in FIG. 2E.

The tractability of the Discrete-Time Transmission Line Model can beenhanced by introducing a noise-whitening filter. Recognizing that thetap-gain coefficients serve as to define the (two-sided) z-transform ofthe sampled autocorrelation function of the received signal at theoutput of the transmission line [Proakis, Equation (6.3.13)], the 2Lroots of the sampled auto-correlation function can be found as shown inFIG. 2F.

The minimum-phase noise-whitening filter is defined by complex rootslying within the unit circle; therefore, the derived equivalentdiscrete-time model of the transmission line [Proakis' Equation(6.3.15)] has the (normalized) tap-gain coefficients.

β^(T)=(0.938 0.327 0.09 0.075)  (1)

The correlation sequence of the equivalent discrete-time model of thetransmission line is given by Proakis' Equation (6.3.17):$\begin{matrix}{x_{k} = {\sum\limits_{n = 0}^{L - {k \div 1}}{a_{n} \cdot \overset{\_}{a_{k + n}}}}} & (2)\end{matrix}$

The channel covariance matrix of the equivalent discrete-time model ofthe transmission line is given by Proakis' Equation (6.4.33):

Γ_(i,j) =if(_(|j·i|) >L,0,if(i<j,x _(|j−i|) ,if(i>j,{overscore(x_(|i−j|)+L )} ,x _(|i−j|) +N ₀)))  (3)

where N₀ is the channel model's noise power spectral density andreciprocal of the receive signal-to-noise ratio. For practical xDLStransmission channels, N₀ is usually negligible; then, for thisillustrative example, $\begin{matrix}\begin{matrix}{x = \begin{pmatrix}1 \\0.343 \\0.109\end{pmatrix}} \\{\Gamma = \begin{bmatrix}1 & 0.343 & 0.109 & 0 & 0 \\0.343 & 1 & 0.343 & 0.109 & 0 \\0.109 & 0.343 & 1 & 0.343 & 0.109 \\0 & 0.109 & 0.343 & 1 & 0.343 \\0 & 0 & 0.109 & 0.343 & 1\end{bmatrix}}\end{matrix} & (4)\end{matrix}$

The channel eigenvalues are determined from Γ to be:

λ^(T)=(0.529 0.598 0.857 1.293 1.723)  (5)

Persons skilled in the telecommunications art will recognize that thepresent invention is applicable to line testing for a number ofdifferent types of lines and for a number of different digital servicesthat such lines may transport. Thus, the inventive testing andcertification techniques readily apply to a wide range of networks.However, to fully appreciate the advantages of the invention, it may behelpful to consider application of the invention to a specific networkfor certification of a specific service, as a detailed example.

FIG. 3 illustrates an example of one type of telephone and data network,which may utilize the line testing and certification in accord with thepresent invention. An example of a Generally similar network isdisclosed in commonly assigned application U.S. Pat. No. 5,790,548.

The end-user may be a single PC user or a small business or aresidential LAN user. The customer access comprises an xDSL twistedpair. In the illustrated embodiment, the network supports standard ADSL,now sometimes referred to as “ADSL-Heavy” and the splitter-less consumerversion of ADSL commonly referred to as “ADSL-Lite”. The ADSL-basedlocal access network provides access to the Internet, to corporate localarea networks (LANs), and the like. The high speeds available throughthe local network enable a wide range of communications, for example, oftext data, of video data, for multimedia, for web browsing, transfers offiles, database searching, and the like.

As shown in FIG. 3, a central office (CO) 100 provides plain oldtelephone service (POTS) and digital subscriber line data service for anumber of customers. For purposes of discussion, assume that theequipment at the various customer premises 200 connect directly to theCO 100 via twisted pair type copper wiring 300. In an actualimplementation, many customers may connect through such wiring to aremote terminal linked to the CO via optical fiber.

For purposes of discussion, the drawing shows three customer premises200. Each customer subscribes to plain old telephone service (POTS). Atcustomer premises 200 ₁ the customer also subscribes to a full ADSLservice, whereas the customer at premises 200 ₂ subscribes to anADSL-Lite service. The network may support a variety of other xDSLservices. Assume, however, that the customer at the lower premises 200 ₃is seeking to add an ADSL service, and the carrier needs to test andcertify a line to those premises for the desired service. As discussedmore later, the test units 165, 265 selectively connect to one or morelines going to those customer premises.

In the CO 100, each customer's line connects to appropriate networkequipment through a main distribution frame (MDF) 101. For telephoneservice, the CO 100 includes a normal POTS switch 103. Since the thirdcustomer currently subscribes only to POTS telephone service, thecustomer's line 300 ₃ connects through the MDF 101 to a line card withinthe normal POTS switch 103. The switch 103 routes calls to and from theline 300 ₃, in the normal manner.

The lines 300 for the other customers, however, connect through the MDF101 to a Digital Subscriber Line Access Multiplexer (DSLAM) 111. TheDSLAM includes a bank of ADSL terminal units 113 and amultiplexer/demultiplexer (MUX) 115. More specifically, within the DSLAM111, each customer line 300 connects to an assigned ADSL terminal unit113 in the central office (ATU-C). In the example illustrated, the firstcustomer's line 300 ₁ connects through the MDF 101 to a first ADSLterminal unit 113 ₁ in the CO 100. The second customer's line 300 ₂connects through the MDF 101 to a second ADSL terminal unit 113 ₂ in theCO 100. The ADSL units 113 include appropriate frequency dependentcombiner/splitters, for segregating out the voice telephone traffic.Thus each ADSL unit 113 provides a connection for telephone traffic fromthe associated line 300 to the POTS switch 103.

Each ADSL terminal unit 113 supports at least the one ADSL service, towhich the customer subscribes. For example, the unit 113 ₂ at leastsupports the ADSL-Lite service. Many xDSL units 113 selectively supporttwo or more services. For example, some vendors units intended for thefull or “heavy” ADSL service can also alternatively send and receive themore limited signals for the “Lite” service.

The ADSL units 113 essentially act as modulator/demodulators (modems)for sending and receiving data over the subscriber telephone lines 300.On the network side, each of the ADSL units 113 connects to the MUX 115.The MUX 115 multiplexes and demultiplexes the upstream and downstreamdata for the ADSL modems 113 and provides a high-speed link a router131.

The router 131 acts as the gateway to a wide-area network illustrated asa data network 132, for example providing packet switched TCP/IPcommunications. The TCP/IP communication may ride on an SMDS network.The SMDS (Switched Multi-Megabit Data Service) network provides fast,packet-switched access to equipment of Internet service providers and toprivate intra-networks operated by corporations and the like. It shouldbe understood that SMDS 13 is simply an example, and that the backbonenetwork 132 may be utilize frame relay or asynchronous transfer mode(ATM).

For the data customer, the network provides a full-time dedicatedconnection intended to be active or “on” at all times. The very firstpacket sent by a customer premise computer goes to the router 131, whichreads the Internet Protocol (IP) address in the packet, determines thatit is desired to set up a session, and commences the steps to establisha session to the appropriate destination through the network 132.

Consider now several examples of customer premises equipment and wiring,for telephone and data services available from the network.

At the customer premises 200 ₁, the copper loop 300 ₁ carrying both thePOTS and ADSL signals connects through a Network Interface Device (NID)201 ₁ placed at the side of the home. A two pair loop is installed fromthe NID to the location where the ADSL unit 203, typically an ATU-Rmodem, is located in the home. One pair connects all of the signals onthe line 300 ₁ from the NID 201 ₁ to the ADSL modem 203. Within theATU-R type modem 203 of the full or “heavy” ADSL service there is apassive splitter/combiner type filter, which segregates the POTS signaland the data signals. The POTS signal is then transmitted over thesecond twisted pair back to the NID 201 ₁. The POTS line is thenconnected to the in-home wiring extensions at the NID 201 ₁, fordistribution to one or more standard telephone devices 205 in the home.

Within the ATU-R type ADSL modem 203, the downstream coded ADSL signalis demodulated and decoded to an appropriate data interface protocol forconnection to the PC 215. The PC 215 also sends data to the ADSL modem203. The modem 203 modulates the upstream data and transmits appropriatesignals over the line 300 ₁ to the modem 113 ₁ in the CO 100. The ATU-Rinterface may support bridging, such that multiple users can share theNIB on the ADSL modem 203, for two-way data communication through the CO100.

At the customer premises 200 ₂, the copper loop 300 ₂ carrying the POTSand ADSL signals again connects through the NID 201 ₂ placed at the sideof the home. For the ‘Lite’ installation, however, there is no need fora splitter and combiner. Both the POTS signal and the ADSL signal aretransmitted over the twisted pair in-home wiring to the ADSL-Lite modem221 and to one or more standard telephone devices 223 in the home.

Within the ADSL-Lite modem 221, the downstream coded ADSL signal isdemodulated and decoded back to an appropriate data interface protocoland supplied to the PC 227. In the upstream direction, the ADSL-Litemodem 221 modulates data for transmission in the appropriate frequencyrange over the twisted pair line 300 ₂ to the ADSL modem 113 ₂ in the CO100.

The customer at premises 200 ₃ presently has only a telephone service.As such, the line 300 ₃ connects through the NID 201 ₃ and the customerpremises wiring to one or more pieces of standard telephone equipment233. The customer may have a computer 235, but for this example, assumethat the computer is not yet connected to the data network. The customerhas asked for an xDSL service, such as ADSL or ADSL-Lite.

The first option is to add the xDSL service to the active subscriber'stelephone line 300 ₃, if practical. Existing loop plant facilitiesmaintained by local telephone carriers typically include additionalcopper pairs that are not in use. Spare wiring typically is available atleast from the NID on the customer premises to the nearest terminal ofthe carrier's feeder cable bundle. In many cases a spare pair shown forexample at 300 ₄ may actually extend all the way through the feedercable(s) and the customer's drop cable to the customer's premises. Toprovide the customer the desired xDSL service, the carrier needs to testand certify one of the wire pairs 300 ₃, 300 ₄ to the customer premises200 ₃.

To test and certify a line, using the illustrated embodiment of theinvention, a carrier technician connects test unit 165 to the centraloffice end of the line under test. Initially, a technician mightmanually connect the test unit 165 at the CO to the CO-end of the line,for example, using jumper connections to the appropriate line terminalson the MDF 101. It is envisioned that future implementations willautomate the connection and operation of the test unit 165 at the CO,for example by incorporation thereof into existing mechanized looptesting (MLT) equipment. A technician may connect the test unit 265 tothe customer premises end of the line. Preferably, the carrier suppliesthe test unit 265 to the customer and instructs the customer to connectthe test unit 265 to the customer end of the particular line 300 ₃.

The two test units communicate test control information, back and forth,to facilitate one or more certification tests of the connected line.Each such test involves one of the units sending a known test patternwaveform over the line to the other unit. The other unit receives,samples and captures a waveform of the signal from the line. The unitreceiving the signal analyzes the waveform of the received signal, toobtain the characteristic eigenvalues. If the eigenvalues bearappropriate relationships to the established thresholds for the desiredservice, the line characteristics are acceptable, and the loop is saidto be “certified.” If the eigenvalues do not show acceptabletransmission characteristics on the line, the carrier has severaloptions. One option is to attempt repairs on the line to improveperformance. Another option is to attempt to find another line to thepremises that is capable of supporting the desired digital service. Aspart of this later solution, the technicians would connect the testunits 165, 265 to the second line 300 ₄ to the premises 200 ₃, as shownby the dotted lines in FIG. 3, and repeat the testing operation. If thesecond line is adequate, the technicians would reconnect the customerpremises wiring at the NID 201 ₃ and the wiring at the MDF 101 toprovide the customer voice telephone service and the xDSL service overthe second line 300 ₄.

FIG. 4 shows the elements of the two test units 165, 265 in somewhatmore detail. Consider first the test equipment 165 at the central office(CO). The test equipment 165 at the CO includes a splitter/combiner 166coupled to the central office end of the line 300 under test. Thesplitter/combiner 166 is essentially similar to that used in an ATU-Ctype central office ADSL modem. For two-way testing of the higherfrequency data channels, the splitter/combiner 166 provides connectionsto a sampling head 167 and a signal generator 169. The sampling head 167digitizes received signals and captures digital data for a waveformsample of the received signals. The signal generator 169 selectivelygenerates precise test waveform signals, for transmission via thesplitter/combiner 166 to the line 300 under test.

The equipment 165 includes a programmable or central processing unit CPU270. The CPU 170 essentially comprises the electronic components of apersonal computer, workstation or the like. The CPU 170 controls alloperations of the test equipment 165 and provides input and outputmechanisms for a user interface for a technician. In an exemplary testprocedure, discussed more below, the CPU 170 serves as the controller ofthe test, providing instructions to the CP test equipment 265 andreceiving results from the CP equipment 265. The CPU 170 includes amicro-processor 171 acting as the programmable control element of theCPU. As such, the micro-processor 171 controls all operations of the CPU170 and thus operations of the test equipment 165.

The main control element of the test equipment 165 is the PC typemicro-processor 171 (e.g. Pentium II), or higher capacity programmableprocessing unit. The sampling head 167 and the signal generator 169connect through an input/output (I/O) port 173 to a communications bus175 of the computer system. The port and bus enable the micro-processor171 to communicate with the signal generator 169 and the sampling head167. For certain tests, the micro-processor may instruct the signalgenerator 169 to send a specific test waveform. For other tests, themicro-processor 171 receives a sample of a received signal waveform fromthe sampling head 167, for further processing in accord with theinvention.

The CPU 170 within the test equipment 165 also includes a hard drive177, for long term program and data storage. At least a portion of thecontrol code for controlling the functions of the micro-processor 171 isstored in a read only memory (ROM) 181. Although not shown, the testequipment 165 also may include a non-volatile memory (EEPROM or Flashmemory) storing programming code that may be modified to upgrade theoperations of the equipment. The test equipment 165 also includes one ormore working memories, such as the random access memory (RAM) 179, cachememory (not shown) and the like. The hard drive and the memoriescommunicate with the micro-processor 171 via the bus 175. For programexecution, program code is loaded from the hard drive 177 to the RAM179. Data files, such as waveform parameters, may be loaded to the RAMwith the program code or retrieved from the hard drive during programexecution.

The computer system of the test equipment 165 also includes a voice-bandmodem 193. The modem 193 communicates with the micro-processor 171 viathe bus 175. The line-side of the modem 193 connects through thevoice-channel coupling of the splitter/combiner 166. The modem 193enables the test equipment to send and receive test control data overthe line 300 to and from the test equipment 265 operating at thecustomer premises.

The test equipment 165 includes a display driver 183 coupled to thecommunications bus 175. In response to instructions from themicro-processor 171, the driver 183 outputs signals to display variousinformation on a monitor 185. The test equipment also includes akeyboard 189 and a cursor control, shown as a track-ball 191 by way ofexample. The keyboard 189 and trackball 191 connect through appropriateinput/output (I/O) interface ports 187 to the bus 175. A technicianoperates the keyboard 189 and/or trackball 191 to input variousinformation to the micro-processor 171. The keyboard, trackball andmonitor represent elements providing a user interface under softwarecontrol by the microprocessor 171. Those skilled in the art willrecognize that the test equipment may incorporate elements providingother user interfaces, such as a touch sensitive LCD screen and/or “softkey” interface. Typically, the software causes the test equipment 165 toimplement a form of graphical user interface.

Although the micro-processor 171 may run other programs, if the computersystem provides other services or functionalities, the control programfor the micro-processor 171 at least includes the routines necessary forproviding the user interface and the routine for controlling the modem193 for data communication with the customer premises test equipment.The control program for the micro-processor 171 also includes theroutines necessary for sending and receiving the various test signals,for analysis of waveforms to derive the representative eigenvalues, andfor comparison of the eigenvalues to reference thresholds.

The monitor 185, keyboard and trackball may be collated with the otherelements of the unit 165, if the unit is essentially built on a PC orworkstation platform. Alternatively only the CPU, signal generator,sampling head and splitter/combiner may be at the CO. The CPU wouldcommunicate with a remote terminal and/or host for purposes of controland user interface. Such a terminal for example might be in a networkoperations center.

The test equipment 265 is generally similar to that of the CO equipment165, except that the customer premises (CP) equipment 265, preferably isbuilt around or connected to a user's computer.

The customer premises test equipment 265 includes a splitter/combiner267 for coupling to the customer premises end of a line 300 under test.The splitter/combiner 267 is essentially similar to that used in anATU-R type remote ADSL modem. For two-way testing of the higherfrequency data channels, the splitter/combiner 267 provides connectionsto a sampling head 269 and a signal generator 271. The sampling head 269digitizes received signals and captures digital data for a waveformsample of the received signals. The signal generator 271 selectivelygenerates precise test waveform signals, for transmission via thesplitter/combiner 267 to the line 300 under test. These three elementsmay be incorporated in a unit designed to plug in-between a user's PCand the line. If the user has purchased a modem capable of the desiredADSL service operations, the modem may serve the functions of theseelements.

The sampling head 269 and the signal generator 271 connect through aninput/output (I/O) port 275 to a communications bus 277. The port andbus enable the micro-processor 273 to communicate with the signalgenerator 271 and the sampling head 269. For certain tests, themicro-processor 273 may instruct the step signal generator 271 to send aspecific test waveform. For another test, the micro-processor 273receives a sample of a received waveform from the sampling head 269, forfurther processing in accord with the invention.

The computer in test unit 265 includes one or more working memories,such as the random access memory (RAM) 282, cache memory (not shown) andthe like. At least a portion of the control code controlling thefunctions of the micro-processor 273 is stored in a read only memory(ROM) 281. Although not shown, the unit 265 also may include anon-volatile memory (EEPROM or Flash memory) storing programming codethat may be modified to upgrade the operations of the test equipment265. For program execution, program code is loaded from the hard drive279 into the RAM 282. Data files, such as waveform parameters andthreshold values, may be loaded to the RAM with the program code orretrieved from the hard drive during test program execution.

Depending on the type of test operations, the equipment 265 may includea number of other elements (not shown). For example, the equipment 165may include a keyboard, a display, a mouse or trackball, and/or aprinter, to provide a user interface and to enable output of reports.Preferably, the test equipment 265 is embodied in the form of a user'sown PC, to which are connected the splitter/combiner, the signalgenerator and the sampling head.

It may be helpful now to go through a step-by-step discussion of aprocedure to test a line, for example for the ADSL service desired bythe customer at premises 200 ₃. Assume at this point that the technicianwants to test the customer's active telephone line 300 ₃. As notedearlier, the technician and or user connect the test units 165 and 265to the respective ends of the line. The technician at the central officeuses the keyboard and/or trackball to control the test procedure.Initially, the two test units establish data communication with eachother, using the modems 183, 293. As the test proceeds, themicro-processor 171 sends commands to in the CP test unit 265 via themodem communication, and the micro-processor 273 may send confirmationmessages and certain test result data back via the modem link.

Assume now that the technician working with the CO test unit 165 firstelects to test the broadband downstream channel capabilities, i.e. forthe expected ADSL operation. The technician inputs data identifying theparticular xDSL service and selects a downstream test. Specifically, thetechnician at the central office or other network location uses thekeyboard 189 and/or the trackball 191 to select the downstream test forADSL. In response, the micro-processor 171 generates a message,identifying the downstream ADSL test. The micro-processor forwards themessage over the bus 175 to the modem 193, and that modem transmits themessage as a modulated data signal through the splitter/combiner 166 tothe telephone line 300. At the customer premises (CP), the modem 283receives the message from the line 300 via the splitter/combiner 267.The modem 283 demodulates the message and supplies it over the bus 277to the micro-processor 273. The micro-processor 273 analyzes thereceived message and determines that the test unit 165 will send a testwaveform over the line 300 to the CP unit. The micro-processor 273initializes the sampling head 269, and sends a “ready” message backthrough the modem link to the micro-processor 171 in the test unit 165at the CO.

The micro-processor 171 retrieves waveform parameter data from storageand sends a command with that data to the signal generator 169, via thebus 175 and the I/O port 173. In response, the signal generator 169outputs a precise test waveform for the upstream channel for theselected ADSL service. For example, to test the downstream transmissionband for an ADSL service, the generator 169 might transmit a white noisesignal having a bandwidth corresponding to that of the band under test.If the ADSL service uses frequency division, the signal band would coverthe frequencies used for all downstream communication. If the ADSLservice uses Echo Cancellation, the test signal band actually mightcover the entire digital communication band, i.e. the downstreambroadband with its overlap of the narrower upstream band. The generator169 supplies the test waveform signal to the downstream data input ofthe splitter/combiner 166, for transmission through the line 300 to thecustomer premises.

The line 300 carries the test waveform signal from the CO to thecustomer premises. The signal will be corrupted to some extent by thecharacteristics of the particular line 300. For example, there may be ageneral attenuation or attenuation in specific frequency portions of thedownstream channel, for example due to the presence of a load coil onthe line. There may be deleterious effects due to excessive capacitanceand resistance on the line, for example due to extended wiring connectedon the circuit by a bridged tap. Alternatively, there may be cross-talkor some other disruptive interference from outside sources. If thecircuit is in relatively good condition, these various effects will beminimal, and the transmitted signal will reach the customer premiseswith little corruption.

At the customer premises, the splitter/combiner 267 passes the spectrumof the downstream ADSL data communications to the sampling head 269. Inthis case, that means that the sampling head 269 receives the testwaveform as corrupted by transport thereof through the customer's line300. The sampling head 269 digitizes samples of the waveform on theline, and captures the digitized samples for a complete waveformrepresentative of the signal received from the telephone line 300. Thesampling head 269 supplies the complete set of digital waveform samplesfor the test to the micro-processor 273, via the I/O port 275 and thebus 277.

The micro-processor 273 could perform the complete eigenvalue analysis.However, if implemented in the user's PC, the carrier has no way to knowin advance if the particular user has a PC with sufficient processorpower to perform the entire calculation in a timely manner. Preferably,the micro-processor performs little or no processing on the digitalsamples and returns data to the CO test unit 165, to complete theprocessing. For example, using the eigenvalue calculation techniqueoutlined above, the CPU in the test unit 265 could be programmed toFourier transform the digitized waveform signals and return thetransform values to the CO test unit. As discussed above, thesetransform values represent the discrete time channel impulse response ofthe line under test with respect to the tested frequency band, in thiscase the downstream channel for ADSL. Alternatively, the test unit 265could simply transmit back the digitized values of the receivedwaveform. The advantage of sending the transform values is that thetransform values represent a relatively small amount of data fortransmission via the low-speed modems as compared to the actual digitalsamples of the received signal. In either case, the micro-processorforwards the data through the bus 277 to the modem 283, which modulatesthe data and sends it through the splitter/combiner 267 and the line 300to the CO test equipment 165.

At the CO, the combiner/splitter 166 supplies the modulated signal tothe modem 193. The modem 193 demodulates the data and forwards it overthe bus 175 to the micro-processor 171. The CPU 170 is programmed insuch a manner that the micro-processor now calculates the eigenvaluesfrom the received data, that is to say, from the raw data or from theFourier transform values. The programming indicates the band offrequencies covered by the particular test, e.g. the ADSL downstreamband, and the program calculates the set of eigenvalues sized to giveappropriate resolution for judging the acceptability of that band.

At this point, the micro-processor 171 can generate signals to produce adisplay of the eigenvalues for the downstream transfer function of theline on the monitor 185. The CPU 170 also is programmed with eigenvaluethresholds for the various communication services. In our example, theeigenvalues represent the transfer function with respect to thebroadband data channel for an ADSL-type communication service. Themicro-processor 171 retrieves the threshold values corresponding to thedownstream ADSL channel from storage. The micro-processor 171 comparesthe threshold values to the eigenvalues derived from the testingoperation. If the eigenvalues bear the desired relationship to thethresholds, for example, if the values exceed all of the thresholds, themicro-processor generates a display of that successful result on themonitor 185.

At this point, if the test for the downstream channel is sufficient forcertification and the test was successful, the technician can certifythe subscriber's line 300 for the desired ADSL service. If a user ison-line at the customer premises, the test unit 165 can send a notice ofcertification over the low-speed modem link, and the user's PC coulddisplay or announce the successful completion of the test.

As noted, the CO test equipment 165 can display the eigenvalues on themonitor 185. The CPU 170 can also provide a variety of software toolsfor analysis of those values, particularly in the situation where thevalues do not meet the thresholds for the desired service. The displaymay show the margin that each value differs from the threshold. Thesoftware may provide displays of corrective options. The technician maybe able to communicate over the carrier's business data network (notshown), to access records regarding the particular line. For example,this communication might allow the technician to determine the nearestterminal connection of the line and the history of past tapping from theline, to thereby help find and eliminate any undesirable bridged taps.

The test units illustrated in FIG. 4 support testing of both downstreamand upstream communications over the subscriber's line. The upstreamtransfer function may be inferred from the downstream test. Theembodiment of FIG. 4 actually enables test in both directions. Forexample, if the line test indicates acceptable capability for thedownstream broadband channel of ADSL, it still may be desirable to testthe upstream channel.

To initiate this second test, the technician operating the CO testequipment 165 uses the keyboard 189 and/or the trackball 191 to selectthe upstream test for ADSL. In response, the micro-processor 171generates a message, identifying the upstream ADSL test. Themicro-processor forwards the test message over the bus 175 to the modem193, and that modem transmits the message as a modulated data signalthrough the splitter/combiner 166 to the telephone line 300. At thecustomer premises, the modem 283 receives the message from the line 300via the splitter/combiner 267. The modem 183 demodulates the message andsupplies it over the bus 277 to the micro-processor 273. Themicro-processor analyzes the received message and determines that thetest unit 265 is to send a test waveform over the line 300 to the CO andrecognizes the service and band involved in the particular test. Themicro-processor 173 may send an acknowledgment back to the test unit 165via the low-speed modems.

The micro-processor 273 retrieves waveform parameter data from storageand sends a command with that data to the signal generator 271, via thebus 277 and the I/O port 275. In response, the signal generator 271outputs a precise test waveform for the upstream channel for theselected ADSL service. In the present example, to test the upstreamtransmission band for an ADSL service, the generator 269 might transmita white noise signal having a bandwidth corresponding to that of theband under test. The generator 271 supplies the test waveform signal tothe upstream input of the splitter/combiner 267, for transmissionthrough the line 300. The line 300 carries the test waveform signal fromthe customer premises to the CO. As with the downstream transmission,the transport of the signal upstream over the twisted wire line in theexample will cause some detectable amount of signal degradation.

At the CO, the splitter/combiner 166 passes the spectrum of the upstreamADSL communications to the sampling head 167. In this case, that meansthat the sampling head 167 receives the test waveform as corrupted bytransport thereof through the customer's line 300. The sampling head 167digitizes samples of the waveform on the line, and captures thedigitized samples for a complete waveform representative of the signalreceived from the telephone line 300. The sampling head 267 supplies thecomplete set of digital waveform samples for the test to the CPU 170.Within the CPU, the digital data for the samples pass through the I/Oport 173 and over the bus 175 to the micro-processor 171, for furtherprocessing. The micro-processor 171 executes the procedure for analyzingthe measured waveform to determine the eigenvalues for the transferfunction of the line 300 with respect to the upstream channel of theADSL service. At this point, the micro-processor 171 can generatesignals to produce a display of the eigenvalues on the monitor 185.

The micro-processor 171 retrieves the threshold values corresponding tothe downstream ADSL channel from storage. The micro-processor 171compares the threshold values to the eigenvalues derived from theupstream testing operation. If the eigenvalues bear the desiredrelationship to the thresholds, for example, exceed all of thethresholds, the micro-processor generates a display of that successfulresult on the monitor 185.

At this point in our example, the system has successfully tested boththe broadband downstream channel and the upstream channel, for theselected ADSL service. The technician can certify the subscriber's line300 for the desired ADSL service. If a user is on-line at the customerpremises, the test unit 165 can send a notice of certification over thelow-speed modem link, and the user's PC could display or announce thesuccessful completion of the test.

As with the downstream test, the test equipment 165 can display theeigenvalues from the upstream test on the monitor 185. The CPU 170 canalso provide a variety of software tools for analysis of any valuesindicating problems with regard to the upstream transmission test.

From the above discussion, it should be apparent that the systems andmethodology of the present invention enable carrier technicians to testthe transmission characteristics of lines, to support digitalcommunications in either direction over a transmission line. Theexamples described enable the technician to certify that the line willsupport a particular grade of xDSL service. If a line does not support adesired service the carrier can take some action. For example thetechnician can test another line. Alternatively, analysis of the linemay show that it supports a lower grade of service that desired by thecustomer. The carrier and the customer may negotiate and agree to use ofthe lower grade DSL service on the line. Alternatively, the carrier caninitiate action to upgrade the line characteristics to support thedesired service and then retest for certification.

The invention also facilitates testing during in-service xDSL operationsover the customer's lines. Over time, line conditions change for manydifferent reasons. A line that initially tested as adequate for aparticular service may deteriorate so that it no longer supports theservice. If a customer reports a trouble with an existing xDSL service,the carrier can retest the line. The eigenvalues determined in the testprocedure provide useful information in determining if the fault isrelated to the line. If the line no longer supports the desired service,again, the carrier may initiate corrective action or may switch thecustomer to another line circuit.

Persons skilled in the art will recognize that a wide range ofenhancements of the exemplary embodiments described in detail above fallwithin the spirit and scope of the concepts of the present invention.For example, the disclosed embodiments utilized specialized testequipment at both ends of the line under test. Future enhancements willrely on the customer's ADSL modem and computer to send test signals overthe line to centralized test equipment operated by the carrier'stechnicians. Alternatively, the digital signal processing and softwarecapability of the ADSL modem and PC might enable the PC to perform aneigenvalue analysis based on a downstream signal from an ATU-C.

As another example, the embodiment of FIG. 4 conducted tests on theupstream and downstream transfer functions by transmitting correspondingtest signals in both directions, exactly the same as if the actual DSLmodems were operating over the line. Alternatively, testing from oneend, preferably the central office end, could analyze the transferffunction for both directions. The characteristic transfer function of atwisted wire pair line is the same in both directions. One approach isto transmit a test waveform covering all bands of interest, e.g that forthe broadband signal and that for the slower upstream signal, in onedirection. This transmission could go from the CO to the CP, or viceversa. The resulting eigenvalues characterize the transfer function withrespect to all frequencies involved in the xDSL service. Anotheralternative is to determine the eigenvalues for the transfer functionfor one direction. e.g. those for the broadband channel, and then inferthe transfer function for the other direction and frequency band fromthe actual test results and knowledge of the characteristics of twistedpair wiring.

While the foregoing has described what are considered to be preferredembodiments of the invention, it is understood that variousmodifications may be made therein and that the invention may beimplemented in various forms and embodiments, and that it may be appliedin numerous applications, only some of which have been described herein.It is intended by the following claims to claim all such modificationsand variations which fall within the true scope of the invention.

What is claimed is:
 1. A method of testing a transmission line for aselected one of a plurality of digital subscriber line services,comprising: selecting one waveform corresponding to the selected digitalsubscriber line service from waveforms corresponding to a plurality ofthe digital subscriber line services; transmitting the selected waveformthrough the transmission line to a receiving end of the transmissionline; sampling and capturing a waveform from the receiving end of thetransmission line; processing samples of the captured waveform to obtaina set of eigenvalues characterizing a transfer function of thetransmission line with respect to the selected digital subscriberservice; and comparing the set of eigenvalues to predetermined thresholdvalues representing an acceptable transfer function of a transmissionline with respect to the selected digital subscriber line service.
 2. Amethod as in claim 1, further comprising determining if each of theeigenvalues bear a predetermined relationship with respect to arespective threshold value, and if so, certifying the transmission linefor the selected digital subscriber line service.
 3. A method as inclaim 2, wherein one eigenvalue bears the predetermined relationshipwith respect to its respective threshold value if the one eigenvalueexceeds its respective threshold value.
 4. A method as in claim 1,wherein the transmission line comprises a telephone line.
 5. A method asin claim 4, wherein the telephone line comprises a subscriber's loop. 6.A method as in claim 5, wherein the subscriber's loop comprises a pairof wires.
 7. A method as in claim 6, wherein the pair of wires comprisesa twisted pair of copper wires.
 8. A method as in claim 1, wherein theplurality of digital subscriber line services comprise xDSL services. 9.A method as in claim 8, wherein the selected digital subscriber lineservice comprises an asynchronous digital subscriber line (ADSL)service.
 10. A method as in claim 9, wherein the ADSL service comprisesa splitterless ADSL service.
 11. A method of testing a transmission linefor a digital subscriber line service, comprising: transmitting a testwaveform signal corresponding to the digital subscriber line servicefrom a transmitting end of the transmission line to a receiving end ofthe transmission line; sampling the waveform signal from the receivingend of the transmission line; processing the sample of the receivedwaveform signal to derive a plurality of eigenvalues characterizing atransfer function of the transmission line with respect to the digitalsubscriber line service; comparing the eigenvalues to threshold valuesrepresenting an acceptable transfer function of a transmission line withrespect to the digital subscriber line service; and from the comparison,determining a relationship of each of the eigenvalues to a respectiveone of the threshold values.
 12. A method as in claim 11, wherein if thedetermined relationships satisfy predetermined conditions, recognizingthat the transmission line is acceptable for the digital subscriber lineservice.
 13. A method as in claim 12, wherein the transmission linecomprises a subscriber's telephone line.
 14. A method as in claim 13,wherein the telephone line comprises twisted pair wiring.
 15. A methodas in claim 14, wherein the digital subscriber line service comprises anasynchronous digital subscriber line (ADSL) service.
 16. A system fortesting a line of a communication network for a digital subscriber lineservice, comprising: a test waveform generator, for connection to atransmit-side of the line, wherein the test waveform generator comprisesa generator for generating a waveform corresponding to the digitalsubscriber line service selected from waveforms corresponding to aplurality of digital subscriber line services available on thecommunication network; a waveform sampler, for connection to areceive-side of the line, for sampling a test waveform received throughthe line from the generator; and a processor, coupled to the waveformsampler and responsive to the sample of the received test waveform, fordetermining a set of eigenvalues characterizing a transfer function ofthe transmission line and for comparing the eigenvalues to thresholdvalues corresponding to an acceptable transfer function of thetransmission line, wherein the processor selects a set of thresholdvalue corresponding to an acceptable transfer function of thetransmission line from a plurality of sets of threshold values, each ofthe sets of the threshold values corresponding to one of the digitalsubscriber line services.
 17. A system as in claim 16, wherein theprocessor comprises a programmable central processing unit.
 18. A linetest unit comprising: a coupler, for connection to a line under test,having an output for a waveform signal received over the line undertest; a waveform sampler responsive to the output of the coupler, forcapturing digital samples of the received waveform signal; a storagedevice storing a plurality of sets of threshold values, each set ofthreshold values characterizing an acceptable transfer function of aline with respect to one of a plurality of digital subscriber lineservices; and a processor coupled to the waveform sampler and to thestorage device, wherein: the processor processes a set of samples for atest waveform corresponding to a selected one of the digital subscriberline services and the captured digital samples, to determine a pluralityof eigenvalues characterizing a transfer function of the transmissionline with respect to the selected digital subscriber line service; andthe processor compares the eigenvalues to a selected one of the sets ofthreshold values, the selected set of threshold values corresponding toan acceptable transfer function for the selected digital subscriber lineservice.
 19. A line test unit as in claim 18, further comprising adisplay responsive to signals from the processor, for generating adisplay of results of the comparison of the eigenvalues to the selectedset of threshold values.